Methods of preparing polysilynes

ABSTRACT

The invention involves new syntheses for poly(methyl- and ethyl-silyne). The invention also includes silicon carbide (SiC) ceramics that can be produced from poly(methylsilyne), as well as other ceramics, which can be produced from these precursors by modified processing conditions.

RELATED APPLICATION

This application is a continuation (and claims the benefit of priorityunder 35 U.S.C. §120) of U.S. application Ser. No. 11/285,372, filedNov. 22, 2005, which claims the benefit of U.S. application Ser. No.10/394,827, filed Mar. 21, 2003, which claims the benefit of U.S.Provisional Application Ser. No. 60/366,851, filed Mar. 22, 2002, thecontents of all of which are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

This invention relates to new methods of preparing poly(methylsilynes)and poly(ethylsilynes).

BACKGROUND

Silicon carbide (SiC) has been used in applications requiring a hard,lightweight, temperature-and-wear-resistant material. SiC has goodfracture strength, hardness, low theoretical density (p=3.21 g/cc) andthus relatively high strength/weight ratio. It is an attractive materialfor numerous applications. Conventionally produced SiC materials aremanufactured using SiC powder processing and sintering. In this process,forming shaped products can be difficult and typically requirestemperatures in excess of 2100° C. A number of chemical approaches basedon polymer precursors have been developed for the synthesis of SiC (seefor example, Laine et al., Chem. Mater. (1993), 5, 260; Richter et al.,Applied Organometallic Chemistry (1997), 11, 71; Seyferth, Adv. Chem.Ser. (1995), 245, 131; and Birot et al. Chem. Rev. (1995), 95, 1443).Polymer precursors can offer some advantages over the conventionalsolid-state processing of SiC. However, some polymers lack the neededdegree of processability or require difficult syntheses. The low charyields of most precursors lead to excessive shrinkage and cracking inthe ceramic products and deterioration of mechanical properties. Theceramics are often rich in either Si or C, which again may lead to adegradation of the desired properties.

Polymethylsilane (PMS) syntheses give high-yield, near stoichiometricSiC ceramics upon pyrolysis. The syntheses involve producing pyrophoricpolymers with the formula (CH₃Si)_(x)(CH₃SiH)_(y), which must be furthercrosslinked by some mechanism (for example, borate (B[OSi(CH₃)₃]₃)thermolysis) to give ceramics in high yield. Chain-terminating agents(such as (CH₃)₃SiCl) have been added to such systems, allowing ceramicyields of up to 64%. Polyvinylsilane has been added to PMS as a furtherpyrophoric ceramic precursor. Stabilization with 2,6di-t-butyl-4-methylphenol (BHT) is generally required for all pyrophoricPMS syntheses. Syntheses of this type tend to be multistep and fairlycomplex.

Polysilynes were synthesized by Bianconi and Weidman in 1988 (Bianconiet al., J. Am. Chem. Soc. (1988), 110, 2342). The synthesis generallyinvolves the reduction of alkyl- or arylsilicon trihalides with liquidNaK. High intensity ultrasound is used to ensure rapid and a morehomogeneous reaction environment. These silicon-silicon bonded networkpolymers adopt a unique structure, in which each silicon bears onependant group and is joined by three single bonds to three other siliconatoms, forming a continuous random network backbone. These siliconnetwork polymers have a distinctive yellow color, very broad NMRresonances, and a broad and intense UV absorption band edge tailing intothe visible. Recently Huang and Vermeulen have synthesized these networkpolymers electrochemically (Chem. Commun. (1998), 247). However, it hasbeen reported that Wurtz coupling of methyltrichlorosilane yields awhite intractable solid, unsuitable for processing into SiC (see Broughet al., J. Am. Chem. Soc. (1981), 103, 3049; West et al., J. Am. Chem.Soc. (1972), 94, 6110; Matyjaszewski et al., Polymer Bulletin (1989),22, 253; Bianconi et al., Macromolecules (1989), 22, 1697; and Vermeulenet al., Polymer (2000), 41(2), 441.

SUMMARY

The invention is based, in part, on the discovery of new methods ofsynthesis for poly(methylsilyne), (CH₃Si)_(n), and poly(ethylsilyne),(CH₂CH₃Si)_(n), and the use of these new methods to make newnon-pyrophoric poly(methyl- or ethyl-silyne) silicon carbide precursors,as well as films and ceramics made from these precursors.

In general, the invention features a novel synthesis of poly(methyl- orethyl-silyne) by a modified Wurtz-type coupling mechanism. The reactionis straightforward and yields a non-pyrophoric polymer, which can beused as an SiC preceramic polymer. The polymer is soluble intetrahydrofuran and other common organic solvents, which enables theformation of films and fibers. The ceramic resulting from the pyrolysisof poly(methyl- or ethyl-silyne) is produced in high yields, and is aperfectly stoichiometric SiC. The SiC ceramic film produced from thePMSy precursor is smooth, continuous, and essentially defect freecompared to films produced by other known methods.

In general, the invention features methods of making poly(methylsilyne)by contacting a halogenated methylsilane with a metallic reagent toproduce a reaction mixture; homogenizing the reaction mixture to producea homogenized reaction mixture; adding to the homogenized reactionmixture a solvent to aid in completing the reaction; refluxing thehomogenized reaction mixture for at least about 6 hours to produce afirst refluxed reaction mixture; contacting the first refluxed reactionmixture with an alkylating agent to produce an end-capped reactionmixture; refluxing the end-capped reaction mixture to produce a secondrefluxed reaction mixture; and quenching the second refluxed reactionmixture with an aqueous solvent that lacks any alcohol to producenon-pyrophoric poly(methylsilyne).

In these methods, the aqueous solvent can be water, the solvent can betetrahydrofuran (THF), the halogenated methylsilane can bemethyltrichlorosilane, the metallic reagent can be a sodium potassiumalloy such as NaK, the alkylating agent can be methylithium, thehalogenated methyl silane can mixed with a non-polar solvent (e.g.,pentane), and ultrasound can be used to perform the homogenization.

In similar methods, poly(ethylsilyne) can be made by contacting ahalogenated ethylsilane with a metallic reagent to produce a reactionmixture; homogenizing the reaction mixture to produce a homogenizedreaction mixture; slowly adding to the homogenized reaction mixture asolvent, wherein at least 1.0 ml of the solvent is added drop-wise, toaid in completing the reaction; adding to the homogenized reactionmixture an alkylating agent to produce an end-capped reaction mixture;and quenching the end-capped reaction mixture with an aqueous solventthat lacks any alcohol to produce non-pyrophoric poly(ethylsilyne).Thus, refluxing steps are not required, but can be included as in themethods of making poly(methylsilynes), and the solvent, such as THF mustbe added slowly. The specific components can be as listed above, exceptthat the halogenated ethylsilane can be ethyltrichlorosilane.

In another aspect, the invention features methods of making ceramics,such as silicon carbides, by forming poly(methylsilyne), orpoly(ethylsilyne), according to the methods described herein; andheating the poly(methyl- or ethyl-silyne) to a temperature of at least200° C. (e.g., at least 500, 750, 1000, 1500, or 1600° C.) to form theceramic. The ceramic can be within 5% of stoichiometric, or can besubstantially stoichiometric silicon carbide, and the poly(methyl- orethyl-silyne) can be heated by exposure to a plasma or a laser. The newceramics can have a mean square roughness of less than 200 Å, scannedover 5 microns or larger scan regions, such as 2 mm.

In another embodiment, the invention also features methods of formingfilms of poly(methyl- or ethyl-silyne) by forming poly(methyl- orethyl-silyne) as described herein; solubilizing the poly(methyl- orethyl-silyne) in a solvent; and coating the solubilizedpoly(methylsilyne) onto a substrate to form a film. The substrate can bealuminum, and the solvent can be tetrahydrofuran.

As used herein “substantially stoichiometric silicon carbide” ismaterial in which the atomic ratio of silicon to carbon is within 5% of1:1.

A non-pyrophoric polymer is one that does not ignite or produce a sparkwhen exposed to air.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. Although methods and materialssimilar or equivalent to those described herein can be used in thepractice or testing of the present invention, suitable methods andmaterials are described below. All publications, patent applications,patents, and other references mentioned herein are incorporated byreference in their entirety. In case of conflict, the presentspecification, including definitions, will control. In addition, thematerials, methods, and examples are illustrative only and not intendedto be limiting.

Other features and advantages of the invention will be apparent from thefollowing detailed description, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a ultraviolet/visible spectrum of poly(methylsilyne).

FIG. 2 is a Fourier Transform Infrared spectrum of poly(methylsilyne).

FIG. 3 is a Fourier Transform Infrared spectrum of poly(methylsilyne)with an extended reflux period and after addition of iodomethane.

FIG. 4 is a ²⁹Si-NMR spectrum of poly(methylsilyne).

FIG. 5 is a graph of % ceramic yield of SiC vs. molecular weight ofpoly(methylsilyne).

FIG. 6 is a photograph of a silicon carbide film produced according to aparticular embodiment of the invention.

FIGS. 7A and 7B are photographs of a prior art silicon carbide filmproduced from a polymethysilane precursor (from Czubarow et al.,Macromolecules, 31:229, 1998).

FIG. 8 is a photograph of a silicon carbide film produced frompoly(n-hexyl)silyne.

DETAILED DESCRIPTION

The invention provides a new and simple modified Wurtz coupling reactionto prepare poly(methyl- or ethyl-silyne), which polymers have previouslybeen impossible to produce using standard Wurtz coupling reactions. Thereaction is straightforward and yields a non-pyrophoric polymer, whichcan be used as an SiC preceramic polymer. The polymer is soluble intetrahydrofuran and other common organic solvents, which enables theformation of films and fibers. The ceramic resulting from the pyrolysisof poly(methyl- or ethyl-silyne) is produced in high yields, and is aperfectly stoichiometric SiC. The SiC ceramic films produced from theprecursors are smooth, continuous, and essentially defect-free.

Methods of Making Poly(Methyl- or Ethyl-Silyne)

Poly(methylsilynes) (PMSy) and poly(ethylsilynes) (PEtSy) are preparedby a modified Wurtz coupling reaction. Suitable starting materialsinclude alkyltrihalosilanes or alkytrialkoxysilanes, such as, forexample, methyltrichlorosilane, methyltrimethoxysilane,ethyltrichlorosilane, or ethyltrimethoxysilane. The starting materialcan be reacted with a reagent used in the known Wurtz reaction such assilver, zinc, activated copper, pyrophoric lead, lithium, complexednickel, potassium, sodium, or cesium metals. Alloys of these metals canalso be used. Suitable reagents include NaK, NaHg, KHg, NaKHg, andsimilar alloys, in any ratio. For example, NaK in an approximately 1:1ratio is suitable for this reaction. A solvent can optionally be used.Nonprotic solvents including a heteroatom such as nitrogen, sulfur, oroxygen can be employed. Such solvents can assist in the formation ofemulsions. Such solvents include ethers, amines, including and notlimited to dimethylsulfoxide, acetonitrile, dimethylformamide, acetone,hexamethylphosphoramide, tetrahydrofuran, diethyl ether, and many othersimilar solvents. Solvents, if used, should be dried thoroughly. Inaddition, when preparing poly(ethylsilyne), the solvent must be addedvery slowly, e.g., drop-wise, for at least the first two milliliters ofthe solvent, to prevent a violent reaction and possible explosion.

The reaction is carried out under an inert atmosphere. Sonication iscarried out to produce a homogeneous mixture, for example, for at leastabout three minutes, e.g., at least about 5, 10, or 15 minutes.

After sonication is complete, the reaction mixture is refluxed under aninert atmosphere for a time suitable to drive the reaction tocompletion, e.g., at least about 6, 8, 10, 12, 18, 20, or 24 hours. Ifthis reflux is not performed in the synthesis of PMSy, a white,intractable solid is formed. On the other hand, such a reflux step hasbeen found to degrade other polysilynes.

After the first reflux step, the polymer is end-capped by exposure to anappropriate amount of an alkylating agent. Such alkylating agentsinclude, for example, alkyllithiums such as methyllithium, and Grignardreagents such as methyl magnesium bromide. After end-capping, themixture is refluxed a second time under an inert atmosphere for afurther period to complete the reaction, for example, for at least 6, 8,10, 12, 18, 20, or 24 hours. Again, if this reflux step is not done inthe synthesis of PMSy, a white intractable solid is formed.

Quenching the reaction after the second reflux step directly with waterproduces the desired yellow PMSy polymer. Handling of the material underan inert atmosphere is no longer required after quenching.Dehalocoupling reactions of halosilanes are typically quenched withmethanol, to consume any unreacted alkali metal. Previously publishedpolysilyne syntheses, and other silicon polymer syntheses, usesequential precipitation from alcohols, including methanol, as apurification step. However, we have found that contact of the polymerwith any alcohol causes instantaneous, irreversible polymer degradationto a white intractable solid, and thus must be avoided in the newmethods described herein.

The polymers produced according to these methods have a molecular weightfrom about 1,000 to about 20,000, e.g., about 1,000, 2,500, 5,000,7,000, 10,000, or 15,000.

Uses of Poly(Methyl- or Ethyl-Silyne)

The polymers can be applied to surfaces using standard coatingtechniques to form silicon carbide (SiC) films. The polymers can beapplied in any desired thickness, by dissolution to any suitableconcentration in any suitable solvent described above. The polymers canalso be spun onto surfaces, or applied to objects according toconventionally known methods. The polymers can also be formed intofibers by using fiber-pulling machines known to those of skill in theart.

SiC formation takes place by heating, or by plasma- or laser-inducedprocesses. For example, heating to at least about 200° C. can lead toforms of SiC that are incompletely crystallized. Higher temperatures,e.g., about 500, 750, 850, 1000, 1200, or about 1500° C., or increasedpressures, e.g., 2 or 3 atmospheres, can produce SiC of a higher degreeof crystallinity. Plasma- or laser-induced SiC formation can take placeat room temperature, since local heating will produce SiC. Lowtemperature SiC formation can also be assisted by adding seed crystalsof SiC.

The performance of ceramic SiC and the applications for which it can beused are very much dependent on ceramic composition. Crystalline formsof SiC can be desirable for use in electronics applications, forexample, for their thermal conduction properties. Incompletelycrystallized forms of SiC can be desirable if films of SiC are to beproduced on substrates which are sensitive to the conditions requiredfor producing substantially crystalline SiC. Films of virtually anythickness can be produced. Fibers can be desirably produced inincompletely crystallized forms. Amorphous forms of SiC can also bedesirable for applications not requiring such demanding physicalproperties.

The highly pure SiC produced by these processes can be used inelectronics applications for protective coatings, and for thermaltransfer applications. Hard drive coatings can be made including SiC.This material can also be incorporated into commonly used items such asboat hulls or tennis racquets. In addition, poly(methylsilyne) can beused to create bulk objects such as molded objects.

Formation of SiC by heat or plasma processing, or under chemicallyreactive atmospheres (such as NH₃, H₂, CH₄ or SiH₄), can be used totailor the new ceramic compositions to form Si_(x)C_(y)N_(z) and canturn PMSy into a ceramic precursor of unrivalled versatility. Processingunder methane, for example, can alter the Si:C ratio further towardcarbon or to create carbon rich Si_(x)C_(y) material. This excesssilicon or carbon is incorporated into the ceramic, and not present aselemental silicon or carbon. Processing under hydrogen gas allowsscavenging of hydride or excess carbon to alter the Si:C ratio towardsilicon. Processing under silane can also alter the Si:C ratio toincrease the silicon content of the material. Processing under ammoniamay introduce Si—N into the silicon carbide, which is generallyconventionally possible by sputtering.

The invention will be further described in the following examples, whichdo not limit the scope of the invention described in the claims.

EXAMPLES

The following examples illustrate particular advantages and propertiesof the materials and methods described herein. All the reactions werecarried out under an argon atmosphere, by means of standard Schlenkmanipulations or inside a glove box. Anhydrous pentane andtetrahydrofuran were purchased from Aldrich and were dried over sodiummetal and benzophenone and distilled prior to their use.Methyltrichlorosilane (99%) was purchased from VWR and used as received.Methyllithium (1.4M in diethyl ether) was purchased from Aldrich andused as received. Liquid 1:1 mole ratio NaK alloy was prepared in aglove box by adding solid potassium to an equimolar amount of moltensodium.

¹H NMR (200.1 MHz) spectra were recorded on a Bruker AC200®. ¹³C NMR(75.4 MHz) spectra were recorded on a Bruker DPX300®. ²⁹Si NMR (99.4MHz) spectra were recorded on a Bruker AMX500®, using a Bruker 5 mmbroadband direct probe. A distortion-less enhanced proton transfer-45(DEPT45) sequence was run with J=7 Hz. In all cases, d⁸-tetrahydrofuranwas used as the solvent at room temperature. FTIR transmission spectrawere obtained using a Midac® M12-SP3 spectrometer, operating at 4 cm⁻¹resolution with neat film samples between salt plates or with KBrpellets. Oxygen incorporation studies were done using a Rayoner® RPR-100photochemical reactor. UVN is spectra were measured at room temperature,in 3×10⁻⁴ M cyclohexane solution using a Shimadzu RUV-260® spectrometer.The molecular weights of the polymers were determined versus polystyrenestandards on a Polymer Labs LC1120® HPLC pump, fitted with an IBM LC9563UV detector, using tetrahydrofuran as a solvent.

As a control, poly(n-hexyl)silyne (PNHS) was synthesized using themethod of Bianconi et al., J. Am. Chem. Soc., 7, 2342 (1988). Pyrolysisstudies of PMSy and poly(n-hexyl)silyne (PnHS) were performed using aThermolyne 12110® tube furnace; all studies were done under a dynamicargon flow and a heating rate of 10° C./minute. Ceramic yields arequoted as percentage weight retention. Films of PMSy and PnHS were spunat 1000 rpm for 10 minutes on AlTiC substrates with an alumina basecoat,on a Headway Research Inc. Photo Resist spinner model 1-EC101DT-435®,from a 0.2 g/mL PMSy/THF solution. Film thickness and roughnessmeasurements were obtained using a Tencor Instruments Alpha Step 500Surface Profiler®. Scanning electron micrographs (SEM) were taken on aJOEL JSM-35CF® scanning microscope. Energy Dispersive X-ray spectroscopy(EDS) was carried out using a JOEL 6320 FXV scanning microscopeconfigured with a PGT Imix Xe X-ray microanalysis system; a 100-secondcollection time was used for X-ray spectral analysis. Spectra were thenquantified as weight percents for Si and C. The XRD pattern was recordedon a Siemens D-500 diffractometer in transmission geometry with a Nifiltered CuKα radiation.

Example 1 Preparation of PMSy

An oven-dried 400 mL beaker containing anhydrous pentane (250 mL) and7.474 g (50 mmol) of methyltrichlorosilane, was placed in a nitrogenatmosphere drybox equipped with a high intensity (475 W, 20 KHz, ½ inchtip) ultrasonic immersion horn. The solvent and methyltrichlorosilanewere irradiated at full power by immersion of the horn for 3 minutes.4.42 g (143 mmol) of NaK alloy was added slowly drop-wise over a periodof 5 minutes. Sonication was continued for a further 8 minutes afteraddition was complete. 200 ml of THF was then added to the reactionmixture, and sonication continued for a further 8 minutes.

At this time the dark blue reaction mixture was transferred to refluxapparatus and transferred to a Schlenk line. The mixture was refluxedgently for 24 hours, under a dynamic flow of argon, in which time thereaction mixture had turned brown in color. At this time, 7.0 mL ofmethylithium (1.4 M in diethyl ether) was added to end-cap the PMSypolymer. Thereafter, reflux was continued for a further 24 hours, againunder a dynamic argon flow. 100 mls of water were added with vigorousstirring to quench the reaction mixture. There was no longer a need foran inert atmosphere at this point. On transferring to a separatingfunnel, separation of the aqueous and organic layers occurred. A yelloworganic layer was isolated from the clear aqueous layer and the solventremoved under vacuum. Yields of 50-70% were typically obtained.

Characterization of poly(methylsilyne) was done by UV/Vis, FTIR, ¹H,¹³C, ²⁹Si NMR spectroscopies, GPC, and by elemental analyses. Asindicated below, all data is consistent with the formation of PMSy. Itshould be noted that PMSy is not pyrophoric, providing a considerableadvantage over many other SiC polymer precursor systems.

FTIR (neat, cm⁻¹ (assignment)): 2973, 2862 (ν C—H, SiCH₃), 2070 (ν Si—H)1460, 1245 (δ C—H, SiCH₃), 1069 (ν Si—O—Si), 911 (γ SiH₂), 836 (ρ CH₃),774, 685 (ν Si—C). ¹³C NMR (ppm assignment): −3.0, very broad, (SiCH₃).²⁹Si NMR (ppm assignment): −74.5 (SiCH₃), −66.3 (HSiCH₃), −37.3 and−33.1 (Si(CH₃)H₂), −21.4 and −15.5 (CH₃SiCH₃ (linear fragments andSi(CH₃)₂ end groups), +8.1 ((CH₃)₃Si). ¹H NMR (ppm assignment): 0.37,very broad (SiCH₃), 3.45, broad, (SiH, SiH₂). Elemental analyses: Found(C, 29.14%; H, 8.37%; Cl, <0.2%); Calculated for (CH₃Si)_(n) (C, 27.85%;H, 7.01%).

The UV/Vis spectrum of PMSy is shown in FIG. 1. It shows a broad andintense absorption in the UV, which tails off into the visible (at about500 nm). This feature is characteristic of polysilynes and is attributedto extension of Si—Si σ-“conjugation” into three dimensions, anddifferentiates polysilynes from linear polysilanes which exhibit strongσ-σ* transitions (λ_(max)=300-350 nm).

The FTIR spectrum of PMSy is shown in FIG. 2. This spectrum isconsistent with that expected for PMSy. It is notable that the ν Si—Hand γ SiH₂ bands are much less intense than in recently reported polymerprecursors to SiC. This is manifested in the fact that the polymer isnot pyrophoric and can be readily handled in air, for short timeperiods. The ν Si—H (2070 cm⁻¹) and γ SiH₂ (911 cm⁻¹) bands can bealmost entirely eliminated by the addition of a couple of mls ofiodomethane to the reaction mixture (after addition of the MeLi) andreflux for a further 24 hours (as shown in FIG. 3). Freshly preparedPMSy also shows relatively few Si—O—Si moieties, as evidenced by thesharp peak at 1069 cm⁻¹. Typically, the presence of large numbers ofSi—O—Si units gives a broad absorption in this region. The absence ofresidual Si—Cl in PMSy as evidenced by the absence of bands in the498-525 cm⁻¹ region is also notable. When handled in air for prolongedtimes and in the direct presence of UV light, PMSy becomes insoluble dueto the incorporation of oxygen into the Si—Si backbone.

The ¹H NMR spectrum (not shown) also confirms that the product is almostentirely PMSy. The resonance at +3.45 ppm (SiH, SiH₂) is very smallcompared to the broad resonance at +0.37 ppm (SiCH₃). Resonances above+5 ppm are not observed; these would be attributable to SiHCl or SiOH.As mentioned, the SiH, SiH₂ signals can be removed by addition ofiodomethane. The ¹³C NMR spectrum (not shown) indicates only thepresence of SiCH₃ as expected.

The ²⁹Si NMR is shown in FIG. 4. The broad resonance at −74.5 ppm isexpected, and is attributable to methyl groups on the silicon backbone.This spectrum also shows the presence of other silicon moieties.

The elemental analysis of PMSy was very close to the expectedcomposition. It shows that the polymer is slightly rich in both carbonand hydrogen, which is to be expected from FTIR, ¹H and ²⁹Si NMR. Cl,<0.2% is consistent with all other data. Si analyses are notoriouslydifficult to obtain from these silicon polymers.

Gel permeation chromatography (GPC) analysis of the polymer revealspolydispersity in PMSy. Generally, we formed polymers of M_(w)≈7000 witha wide polydispersity (≈4). We also formed polymers with molecularweights up to about 20,000, and with polydispersity of about 7. We havealso formed brown insoluble powders, and these may be even highermolecular weight versions of PMSy, which would account for theinsolubility.

Pyrolysis studies confirm that that the major weight loss process forPMSy occurs in the region 200-450° C. The ceramic yield is very muchdependent on the molecular weight of the polymer, as shown in the graphin FIG. 5, which shows the percentage ceramic yield of SiC vs. molecularweight of poly(methylsilyne). In approximately 50% of the pyrolysedsamples a black-colored ceramic is obtained. However, in the remaininghalf of the samples the ceramic displays light brown to pale yellowcoloration. This coloring is indicative of extremely high purity siliconcarbide, as observed by Greenwood and Earnshaw, Chemistry of theElements, Pergammon Press, New York, (1989), 386.

Example 2 Preparation of PEtSy

An oven-dried 400 mL beaker containing anhydrous pentane (250 mL) and7.474 g (50 mmol) of ethyltrichlorosilane, was placed in a nitrogenatmosphere drybox equipped with a high intensity (475 W, 20 KHz, ½ inchtip) ultrasonic immersion horn. The solvent and ethyltrichlorosilanewere irradiated at full power by immersion of the horn for 3 minutes.4.42 g (143 mmol) of NaK alloy was added slowly drop-wise over a periodof 5 minutes. Sonication was continued for a further 8 minutes afteraddition was complete. 200 ml of THF was then added to the reactionmixture very slowly, e.g., drop-wise for the first 2 or 3 mL, andsonication continued for a further 8 minutes.

At this time the dark blue reaction mixture was transferred to refluxapparatus and transferred to a Schlenk line. The mixture was(optionally) refluxed gently for about 12 hours, under a dynamic flow ofargon, during which time the reaction mixture had turned brown in color.At this time, 7.0 mL of methylithium (1.4 M in diethyl ether) was addedto end-cap the PEtSy polymer. Thereafter, reflux was continued for afurther 24 hours, again under a dynamic argon flow. 100 mls of waterwere added with vigorous stirring to quench the reaction mixture. Therewas no longer a need for an inert atmosphere at this point. Ontransferring to a separating funnel, separation of the aqueous andorganic layers occurred. A yellow organic layer was isolated from theclear aqueous layer and the solvent removed under vacuum. Yields of50-70% were typically obtained.

Example 3 Preparation of Silicon Carbide (SiC)

PMSy heated to pyrolysis temperatures of 1000° C., under argon, producedSiC in high yield (up to 85%, by weight loss, which is close to thetheoretical yield expected for this polymer). During heating, thetemperature was slowly ramped up at a rate of 10° C./minute, and held at250° C. for about 2 hours before continuing to increase the temperature.

The ceramic yield is very much dependent on the molecular weight of thepolymer, which is a well-known attribute of these types of polymers.Purity is confirmed by elemental analysis of the ceramic: C, 27.30; Si,61.20; Σ=88.50; Calcd for SiC: C, 29.95; Si, 70.05. This equates to SiCwith the formula SiC_(1.04) or a ceramic with the composition 1.08%C+98.92 SiC. Energy dispersive spectroscopy (EDS) analysis of theceramic film formed from PMSy on the alumina substrate reveals the highpurity of the ceramic product: Found C, 29.95%; Si, 70.05%; Calcd forSiC: C, 29.95; Si, 70.05, or a ceramic with the composition of 100% SiCwith Si and C present in a perfect 1:1 ratio. Such analytically pureceramic has not been obtained from use of other polymer precursorsystems.

Example 4 Preparation of PMSy Films

Samples of PMSy were spun onto alumina substrates to obtain uniform andsmooth films of PMSy (2 μm thick, mean square roughness (Rq)=200-300 Å,scanned over 2 mm). Heating these films to 1000° C. produced smoothceramic films of uniform thickness (1 μm thick, Rq=170 Å, scanned over 2mm), as measured by. A photograph of this material is shown in FIG. 6.The smoothness indicates a dense, homogeneous ceramic film (in the areascanned), without pores, cracks, or other defects; such high-qualityceramic is not reported from use of other polymer precursor systems,absent further polishing. The ceramic films produced were adherent tothe substrates, resistant to removal by plastic adhesive tape, and werecompletely uniform.

Example 5 Comparisons with Other Films

FIGS. 7A and 7B are published photographs of an SiC film made from apolymer precursor (polymethylsilane) heated to 1000° C., under argon. Asshown, the film had significant defects and was not continuous. Thefigures are from Czubarow et al., Macromolecules, 31, 229, (1998).

The improved ceramic-producing behavior of PMSy over other polysilynes,such as poly(n-hexyl)silyne, is shown in FIG. 8. A film ofpoly(n-hexyl)silyne is shown after being spun onto an alumina substrateand pyrolysed. Severe cracks and inhomogeneities are seen in theresulting ceramic film. This is believed to result from loss of most ofthe mass of the n-hexyl side chain during pyrolysis, leading to highweight loss and low ceramic yield.

OTHER EMBODIMENTS

It is to be understood that while the invention has been described inconjunction with the detailed description thereof, the foregoingdescription is intended to illustrate and not limit the scope of theinvention, which is defined by the scope of the appended claims. Otheraspects, advantages, and modifications are within the scope of thefollowing claims.

1. A method of making a poly(methylsilyne), the method comprising: a)contacting a halogenated methylsilane with a metallic reagent to producea reaction mixture; b) homogenizing the reaction mixture to produce ahomogenized reaction mixture; c) adding to the homogenized reactionmixture a solvent to aid in completing the reaction; d) refluxing thehomogenized reaction mixture for at least about 6 hours to produce afirst refluxed reaction mixture; e) contacting the first refluxedreaction mixture with an alkylating agent to produce an end-cappedreaction mixture; f) refluxing the end-capped reaction mixture toproduce a second refluxed reaction mixture; and h) quenching the secondrefluxed reaction mixture with an aqueous solvent that lacks any alcoholto produce non-pyrophoric poly(methylsilyne).
 2. The method of claim 1,wherein the aqueous solvent is water.
 3. The method of claim 1, whereinthe solvent is tetrahydrofuran.
 4. The method of claim 1, wherein thehalogenated methylsilane is methyltrichlorosilane.
 5. The method ofclaim 1, wherein the metallic reagent is a sodium potassium alloy. 6.The method of claim 1, wherein the alkylating agent is methylithium. 7.The method of claim 1, wherein the halogenated methyl silane is mixedwith a non-polar solvent.
 8. The method of claim 7, wherein thenon-polar solvent is pentane.
 9. The method of claim 1, whereinultrasound is used to perform the homogenization.
 10. A method of makinga poly(ethylsilyne), the method comprising: a) contacting a halogenatedethylsilane with a metallic reagent to produce a reaction mixture; b)homogenizing the reaction mixture to produce a homogenized reactionmixture; c) slowly adding to the homogenized reaction mixture a solvent,wherein at least 1.0 ml of the solvent is added drop-wise, to aid incompleting the reaction; d) adding to the homogenized reaction mixturean alkylating agent to produce an end-capped reaction mixture; and e)quenching the end-capped reaction mixture with an aqueous solvent thatlacks any alcohol to produce non-pyrophoric poly(ethylsilyne).
 11. Themethod of claim 10, wherein the aqueous solvent is water.
 12. The methodof claim 10, wherein the solvent is tetrahydrofuran.
 13. The method ofclaim 1, wherein the halogenated ethylsilane is ethyltrichlorosilane.14. The method of claim 1, wherein the metallic reagent is a sodiumpotassium alloy.
 15. The method of claim 1, wherein the alkylating agentis methylithium.
 16. The method of claim 1, wherein the halogenatedethylsilane is mixed with a non-polar solvent.
 17. The method of claim16, wherein the non-polar solvent is pentane.
 18. The method of claim 1,wherein ultrasound is used to perform the homogenization.
 19. The methodof claim 10, further comprising refluxing the homogenized reactionmixture.
 20. The method of claim 10, further comprising refluxing theend-capped reaction mixture.
 21. A method of making a ceramic, themethod comprising forming poly(methylsilyne) by the method of claim 1;and heating the poly(methylsilyne) to a temperature of at least 200° C.to form the ceramic.
 22. The method of claim 21, wherein thepoly(methylsilyne) is heated to at least 1000° C.
 23. The method ofclaim 21, wherein the ceramic is within 5% of stoichiometric.
 24. Themethod of claim 21, wherein the poly(methylsilyne) is heated by exposureto a plasma.
 25. The method of claim 21, wherein the poly(methylsilyne)is heated by exposure to a laser.
 26. The method of claim 21, whereinthe ceramic is silicon carbide.
 27. The method of claim 21, wherein theceramic has a mean square roughness of less than 200 Å, scanned over 5microns.
 28. A method of making a ceramic, the method comprising formingpoly(ethylsilyne) by the method of claim 1; and heating thepoly(ethylsilyne) to a temperature of at least 200° C. to form theceramic.
 29. The method of claim 28, wherein the poly(ethylsilyne) isheated to at least 1000° C.
 30. A method of forming a film ofpoly(methylsilyne), the method comprising forming poly(methylsilyne) bythe method of claim 1; solubilizing the poly(methylsilyne) in a solvent;and coating the solubilized poly(methylsilyne) onto a substrate to forma film.
 31. The method of claim 30, wherein the solvent istetrahydrofuran.
 32. A method of forming a film of poly(ethylsilyne),the method comprising forming poly(ethylsilyne) by the method of claim10; solubilizing the poly(ethylsilyne) in a solvent; and coating thesolubilized poly(ethylsilyne) onto a substrate to form a film.
 33. Themethod of claim 32, wherein the solvent is tetrahydrofuran.